Introduction

Case Study

Observation - conjecture - test - understanding - consequences

An unexpected emergent phenomenon

During the summer, the phytoplankton biomass shows an unexpected fifteen-day oscillation in the mixed layer (see "Emergent Population Ecology in a Virtual Ecosystem"). Further investigation reveals correlated variation in ammonium concentration and grazing rate. Analysing the zooplankton reveals a correlated modulation of the depth of nocturnal feeding, which alternates between the mixed layer and the deep chlorophyll maximum.

Conjecture

It is conjectured that the fifteen-day oscillation is caused by the following sequence of events:

1. zooplankton graze on phytoplankton in the mixed layer until the concentration is below their ingestion threshold,
2. they then switch to feeding on phytoplankton in the deep chlorophyll maximum,
3. while feeding, the zooplankton release nitrogen into the water; this fertilizes primary production,
4. the concentration of phytoplankton in the mixed layer rises while the zooplankton are away feeding in the deep chlorophyll maximum,
5. after about one week the zooplankton migrate back to the mixed layer to graze on the new crop of phytoplankton.

Then the cycle repeats. The amplitude declines slowly because some nitrogen is lost at every cycle..

This is an example of an emergent phenomenon in the virtual ecosystem, which depends on a complex interaction of many basic processes.

Audit trails

We start to test our hypothesis by examining the audit trails of individual zooplankton. These confirm the crucial aspects in the process:

(1) The trajectory of the selected animal shows its diel migration, and nocturnal foraging on phtoplankton, alternating between the mixed layer and the deep chlorophyll maximum on a 15-day cycle.

(2) The zooplankton releases nitrogen into the mixed layer, either directly (by excretion), or indirecting (as faecal pellets, which are remineralized by bacteria).

(3) Plotting the history of a faecal pellet shows microbial action releasing its nitrogen into the mixed layer.

A numerical experiment

Fertilization of primary production in the oligotrophic mixed layer depends on an upward flux of nitrogen carried by migrating zooplankton. The model can easily be changed to stop this upward flux. Fertilized production in the mixed layer then stops. The seawater becomes less turbid. That changes the depth of the oligotrophic mixed layer; in particular its deepening in autumn. The date of the end of summer oligotrophy (marked by the autumn bloom) is delayed by ten days when zooplankton fertilization is turned off.

Passive tracer

A similar upward flux should exist for any biologically-passive chemical that is taken up by phytoplankton at the same rate as nitrogen. To test that hypothesis we create a new virtual ecosystem which includes a passive chemical, which can occur in solution, in all organisms (dead or alive) and in faecal pellets. The scenario includes an event by which the chemical is injected into the seasonal thermocline at a depth of 40m. The chemical is taken up by phytoplankton in the deep chlorophyll maximum, then by zooplankton which graze on them. Soon the chemical appears in the mixed layer. Detailed analysis reveals that the buildup in the mixed layer is due mainly to microbial action on the faecal pellets of carnivores that ate some of the herbivores. A smaller contribution comes from herbivore excretion and from microbial action on the herbivore faecal pellets. So primary production in the oligotrophic layer depends mainly on the death rate of the herbivores in the mixed layer.

Significance

1. Summer variation in mixed layer depth is influenced by zooplankton.
Fertilization advances the autumn bloom by ten days

2. The phytoplankton population nourished by zooplankton seeds the spring bloom next year..
So the mechanism is important for inter-annual population dynamics.